使用Lambda删除重复项

本文关键字：删除 Lambda 使用 | 更新日期: 2023-09-27 18:27:16

我需要一些帮助，使用lambda表达式来删除实体框架上下文中的重复项。我有一张表，上面有以下几列：

Id, DateOfIncident, Description, EmployeeId, IncidentTypeId, and IsAttendanceIncident

我想删除DateOfIncident, EmployeeID, IncidentTypeID and IsAttendanceIncident are the same.中的重复条目。我确实想保留一个条目。我知道如何使用SQL中带有CTE的存储过程来完成这项任务，但我不知道如何使用Lambda表达式来完成这一任务。

这段代码返回一个列表，其中不包括我的重复项，但现在我该如何删除不在此列表中的重复项？

  var query = db.Incidents.Where(x => x.IsAttendanceIncident == "Y").GroupBy(x => new { x.EmployeeId, x.DateOfIncident, x.IsAttendanceIncident })
         .Select(x => x.FirstOrDefault());

更新：

所以我继续写自定义IEqualityComparer。现在，我如何使用id删除上下文中不在distinctItems中的事件？

 static void Main(string[] args)
    {
        DALIncidents.AttendanceEntities1 db = new DALIncidents.AttendanceEntities1();
       IEnumerable<DALIncidents.Incident> distinctItems = db.Incidents.Where(c => c.IsAttendanceIncident == "Y");
       distinctItems = distinctItems.Distinct(new DALIncidents.DistinctIncidentComparer());
       Console.ReadLine(); 
    }

使用Lambda删除重复项

var query = db.Incidents
    .Where(x => x.IsAttendanceIncident == "Y")
    .GroupBy(x => new { x.EmployeeId, x.DateOfIncident, x.IsAttendanceIncident })

示例1:

    .Select(x => x.FirstOrDefault());  // your original code which retrieves entities to not delete
var dupes = db.Incidents.Except( query ); // get entities to delete

示例2:

    .SelectMany( x => x.OrderBy( y => y.Id ).Skip(1) ); // gets dupes directly
var dupes = query; // already have what we need

最后：

foreach( var dupe in dupes )
{
    db.Incidents.Remove( dupe );
}

从我之前使用的测试上下文生成的示例SQL，其中Person实体与手表有1:N关系：

C#：

context.Persons.SelectMany(x => x.Watches.OrderBy(y => y.Id).Skip(1))

生成的SQL:

SELECT 
1 AS [C1], 
[Skip1].[Id] AS [Id], 
[Skip1].[Brand] AS [Brand], 
[Skip1].[Person_Id] AS [Person_Id]
FROM  [dbo].[Persons] AS [Extent1]
CROSS APPLY  (SELECT [Project1].[Id] AS [Id], [Project1].[Brand] AS [Brand], [Project1].[Person_Id] AS [Person_Id]
    FROM ( SELECT [Project1].[Id] AS [Id], [Project1].[Brand] AS [Brand], [Project1].[Person_Id] AS [Person_Id], row_number() OVER (ORDER BY [Project1].[Id] ASC) AS [row_number]
        FROM ( SELECT 
            [Extent2].[Id] AS [Id], 
            [Extent2].[Brand] AS [Brand], 
            [Extent2].[Person_Id] AS [Person_Id]
            FROM [dbo].[Watches] AS [Extent2]
            WHERE [Extent1].[Id] = [Extent2].[Person_Id]
        )  AS [Project1]
    )  AS [Project1]
    WHERE [Project1].[row_number] > 1 ) AS [Skip1]

var query = db.Incidents.Where(x => x.IsAttendanceIncident == "Y")
                .GroupBy(x => new { x.Id, x.EmployeeId, x.DateOfIncident, x.IsAttendanceIncident })
                .Select(x => x.FirstOrDefault());

var query2 = from duplicate in db.Incidents
                 .Where(x => x.IsAttendanceIncident == "Y" && !query.Any(i => i.Id == duplicate.Id));

query2现在只包含重复项？

您将需要使用Distinct函数，如果您只想要一些字段，则需要创建一个Equality Comparer。（IEqualityComparer）

Ahh刚刚看到上面的评论，查看更多：

使用linq 删除列表中的重复项

var query = db.Incidents.Where(x => x.IsAttendanceIncident == "Y").GroupBy(x => new { x.EmployeeId, x.DateOfIncident, x.IsAttendanceIncident })
     .SelectMany(x => x.Skip(1));
相关文章：